Velocity-selective (VS) pulse trains can provide unique functionalities when designing pulse sequences for various Magnetic Resonance Imaging (MRI) based hemodynamic evaluation: MR angiography (MRA); blood flow (perfusion), blood volume, or transit time; oxygen extraction fraction; metabolic rate of oxygen.
Fourier-transform based VS magnetization-prepared MRA has been introduced for visualization of vessels based on the designated flow velocity and allows for a large spatial coverage. Specifically, the angiographic signal is achieved, by setting the flowing spins in the pass-band and static spins in either the inversion-band or saturation-band.
The combination of non-selective RF pulse trains with embedded velocity-encoding gradients, based on the Fourier-transform, can produce almost arbitrary velocity-selective profiles. However, the original scheme (without refocusing pulses) suffers from off-resonance effect which is manifested as excitation profile shifting along velocity. The susceptibility to B0 field inhomogeneity can be alleviated, by incorporating one composite refocusing pulse within each velocity encoding step and modifying the RF and gradient waveforms accordingly, as recently shown for peripheral MRA at 1.5 T. However, the tolerable B0 offset is limited to ±80 Hz and the sensitivity to B1 inhomogeneity remains an issue particularly at high field strength. Unfaithful B1+ scale (ratio of actual flip angle to nominal input flip angle) leads to two independent consequences: incorrect RF weighting for the excitation k-space by the hard pulse at the beginning of each velocity encoding step and thus inaccurate flip angle for either the inversion or saturation band; imperfect refocusing during each velocity encoding step and thus degraded velocity selective profile at off-resonance.
It would therefore be advantageous to provide an extended velocity-selective pulse train designed with more robust insensitivity to both B0 and B1 field inhomogeneity and eddy currents.